doc: fix link in the test/README.md

PR-URL: #15642
Reviewed-By: Bryan English <bryan@bryanenglish.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/README.md

@@ -6,7 +6,7 @@ For a detailed guide on how to write tests in this
 directory, see [the guide on writing tests](../doc/guides/writing-tests.md).
 
 On how to run tests in this directory, see
-[the contributing guide](../CONTRIBUTING.md#step-5-test).
+[the contributing guide](../CONTRIBUTING.md#step-6-test).
 
 ## Test Directories